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Explorer 39 (Air Density experiment C, or AD-C) was an inflatable sphere, 3.6 m in diameter. It was orbited to make atmospheric density determinations. The spacecraft was successfully launched into a nearly polar, highly elliptical orbit. It was folded and carried into orbit, together with ejection and inflation equipment, as part of the payload of Explorer 40. Two density experiments were performed. One involved the study of systematic density variation, and the other was concerned with nonsystematic density changes. The upper atmospheric densities were derived from sequential observations of the sphere by use of an attached 136.620-MHz radio tracking beacon and by optical tracking. The radio beacon ceased transmitting in June 1971. Since that time it has been necessary to rely solely on the SAO Baker-Nunn camera network for tracking. Explorer 39 has an expected orbital lifetime of 50 years.

Alternate Names

  • 03337
  • Air Density experiment C
  • Explorer 39
  • PL-683J

Facts in Brief

Launch Date: 1968-08-08
Launch Vehicle: Scout
Launch Site: Vandenberg AFB, United States
Mass: 9.4 kg

Funding Agency

  • NASA-Office of Space Science Applications (United States)


  • Earth Science

Additional Information

Questions and comments about this spacecraft can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Mr. John R. HoltzProgram ManagerNASA Headquarters
Mr. Claude W. Coffee, Jr.Project ManagerNASA Langley Research Center
Dr. Robert F. FellowsProject ScientistNASA Headquarters
Dr. Gerald M. KeatingGeneral ContactNASA Langley Research Center
Dr. Erwin R. SchmerlingProgram ScientistNASA Headquarters
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